In completion of a well, one process presently in favor is the use of a tubing conveyed perforating assembly suspended on a tubing string to form perforations at a specified depth in the well. The TCP process typically involves suspending a set of perforating guns (ranging from a few to several hundred) at the lower end of the tubing string. The tubing string is assembled at the well head and lowered into the well. The TCP assembly is guided by a packer to register the TCP assembly opposite the formation of interest prior to forming the perforations. Ordinarily, a detonating bar is dropped free fall in the tubing string. The bar strikes the top end of the apparatus with the TCP assembly thereby triggering detonation. The detonating bar normally weighs quite a bit. Moreover, the tubing string can be quite long, easily more than 10,000 feet, and the bar may well reach significant velocity as it falls into the well. If the bar falls freely without impediment, it will travel with sufficient kinetic energy that it may do damage to the equipment at the top end of the TCP assembly. Because of this, it is desirable to retard the rate of fall of the detonating bar. One way to do this is to place a standing column of liquid above the TCP assembly so that the detonating bar is retarded by the liquid. This regulates detonating bar velocity to assure that the kinetic energy in the impact is in an acceptable range.
One problem which makes detonating bar velocity variable is a change in viscosity of the fluid in the tubing string. Assume as an easy example that the tubing string is filled with certain depth with clean water. This will provide a known retardation to the velocity of the drop bar. On the other hand, if the water mixes with drilling fluids or formation fluids or both, it can easily become quite different in physical characteristics and thereby provide significantly different retardation to the velocity of the detonating bar. It is therefore desirable to limit commingling of the fluids so that drilling fluids or formation fluids on the exterior of the tubing string do not invade the string and thereby change the viscosity of the standing column of liquid. It is particularly possible to mix drilling fluid in the water and thereby significantly change the retardation of the water to the dropped detonating bar.
The present apparatus enables isolation of the standing column of water above the TCP assembly. Moreover, there maybe variations in downhole pressure. The present apparatus accommodates pressure differentials between the column of standing fluid above the TCP assembly and the exterior in the annulus of the well. Briefly, this apparatus includes a floating piston assembly which is enclosed in a suitable sub. The piston assembly can ride up and down to achieve a pressure balance. The floating piston assembly is sealed over by glass disk. When the detonating bar is dropped, it shatters the glass disk and passes through it. The sacrificial glass disk isolates fluid therebelow to assure that that fluid is clean. Thus, the floating piston assembly rises and falls for clean fluid isolation. Moreover, should pressure increase below the glass disk, an O-ring valve assembly vents fluid in one direction only, thereby accomplishing controllable pressure relief, all as will be set forth in detail herein after.